This invention relates generally to a system for recording and/or reproducing digital information on an optical medium, and, more particularly, to a system which performs a high-speed search for the information stored on the optical medium.
Information is generally stored by an optical disc in the form of concentric or spiral tracks sometimes referred to as information tracks. A recording and/or reproducing device rotates the optical disc while using a light beam to retrieve the information from or record the information to the optical disc. As the optical disc rotates, the light beam radially traverses the optical disc while a tracking servo loop in the recording and/or reproducing device keeps the beam of light centered on the information track, or, alternately, the track will become the information track in the case of recording information to the optical disc.
A three-beam arrangement is one common arrangement used to supply tracking signals to a tracking servo loop, which is maintaining the light beam on the current track of the optical disc. In this arrangement, a laser beam passes through a diffraction grading to form a center beam and two secondary beams. The center beam is used to read or record information on the optical disc and the two secondary beams are used for tracking the current track on which the information is being read or recorded. The two secondary beams form two spots on opposite sides of a track offset with respect to each other. A photodetector array includes a main array of four photodetector sensing the reflection of the center beam and two individual photodetectors, commonly referred to as the E and F photodetectors, sensing light from the two side beams reflected off of the optical disc.
A xe2x80x9csearchxe2x80x9d or xe2x80x9cseekxe2x80x9d operation is a common operation of a recording and/or reproducing device the purpose of which is to move the light beams from the current track, i.e., the track wherein the light beam is presently positioned, to a target track. During the xe2x80x9csearchxe2x80x9d operation, the recording and/or reproducing device typically searches for the target track on the optical disc. Achieving a search operation may require the light beams to radially move across several information tracks starting from the current track before the target track is found. Once the target address is found, the optical disc storage device can return to its normal mode of retrieving or recording information.
One method of search is to estimate based on the starting location of the light beam and the physical parameters of the movement system (e.g., mass of a carriage drive, velocity of movement of an optical head), where the target track might be located, and to then initiate commands to move the optical head which controls the light beams toward this target track. The optical head is then moved to the estimated track. The estimated track is read to determine if the track has been reached. If the target track has been overshot or undershot a further estimate is made and the optical head is again moved in a direction toward the target track. These steps are then repeated until the target track is reached. The disadvantage of this approach is that it is slow because each time a track is read a close loop tracking operation must be achieved.
Another method of search is to employ a counter that keeps an accumulated total of the number of tracks crossed as the optical head is moved radially across the disc. The optical head then moves towards the target track a number of tracks determined to be the absolute value of the starting track number subtracted from the target track number. This approach speeds up the search but it is only as effective as the accuracy of the track crossing counter. Previous counting devices have counted the total number of tracks traversed whether a track is traversed in a forward or backward direction. In the situation where an optical disc is subject to vibrations and acceleration forces, the optical head may move back and forward several times from the initial track to the destination track. Thus, the total number of tracks counted will be an overestimate of the actual number of tracks traversed. An additional source of error in the count may be introduced due to eccentricity of the optical disc. An inaccurate count track slows down the search, since if the target track has been overestimated or underestimated a new search must be initiated in order to move the optical head to the target track.
In order to solve the above problems, it is desirable to find an apparatus and method for counting tracks during a search, which is accurate even in the presence of vibration, acceleration forces, eccentricity, and other sources of error.
In view of the foregoing, an object of the present invention is to provide an optical information recording/reproducing apparatus that is capable of searching a target track with an enhanced reliability.
Accordingly, another object of the present invention is to provide an optical information recording/reproducing apparatus which is capable of searching a target track with an enhanced reliability and accuracy during a search regardless of the presence of vibrations or other acceleration forces acting on the recording/reproducing apparatus.
Accordingly, another object of the present invention is to provide an optical information recording/reproducing apparatus that is capable of a high-speed search.
It is another object of the present invention to provide a high-speed search having improved accuracy of track counting during the high-speed search.
It is yet another object of the present invention to provide a search apparatus and method with improved accuracy of track counting during a high-speed search by taking into account the direction of crossing of a track during the search.
It is still another object of the present invention to provide a search apparatus and method with improved accuracy of track counting during a high-speed search by counting the net track movement, rather than counting the total accumulated movement resulting from bidirectional track crossings.
It is yet another object of the present invention to provide a search apparatus and method with improved accuracy of track counting during a high-speed search by arranging the E and F beams of a three-beam tracking system in quadrature, to thus enable a high-speed search based on an accurate track count using simplified components.
It is still another object of the present invention to provide a high-speed search by using a plurality of rates of motion.
It is yet another object of the present invention to provide a high-speed search by using a plurality of rates of motion and choosing one of the rates of motion based on an improved track count of the present invention.
Further objects and advantages of the present invention will become apparent from a consideration of the drawings and ensuing description.
In order to achieve the above-mentioned objectives, the present invention conducts a high-speed search on an optical medium having a plurality of tracks on which information is recorded. At least a first light spot and a second light spot are directed by an optical system onto the optical medium. The light spots traverse across the tracks in one of a first direction and a second direction. A photodetector unit receives a reflected component of the first light spot and a reflected component of the second light spot to form, respectively, a first electrical signal and a second electrical signal. Digital shaping circuitry respectively converts the first electrical signal and the second electrical signal into a first digital signal and a second digital signal. A quadrature detector receives the first digital signal and the second digital signal to produce from the first digital signal and the second digital signal an up-count signal indicating that the light spots are traversing the tracks in the first direction and a down-count signal indicating that the light spots are traversing the tracks in the second direction.
According to another aspect of the present invention, a counter counts, during the search, the up-count signal and the down-count signal to determine a number of tracks traversed by the light spots.
In yet another aspect of the present invention, a microcomputer is coupled to the quadrature detector and is configured to count, during the search, the up-count signal and the down-count signal to identify a number of tracks traversed by the light spots.
In an additional aspect of the present invention, the first light spot and the second light spot are arranged on the tracks in a quadrature relationship to each other. In an additional embodiment of the present invention, the first electrical signal and the second electrical signal are arranged on the tracks in a quadrature relationship to each other. In another embodiment of the present invention, the first digital signal and the second digital signal are arranged in a quadrature relationship to each other.
In a further aspect of the present invention, the quadrature relationship is characterized by about a 90-degree shift between the first digital signal and the second digital signal.
In accordance with yet another aspect of the present invention, the quadrature relationship is characterized by a tolerance relationship between the first digital signal and the second digital signal. The tolerance relationship is determined so that the first digital signal and the second digital signal vary within a specified number of degrees of 90 degrees as permitted by a tolerance parameter of the quadrature detector.
In still another aspect of the present invention, the quadrature relationship is characterized by the first digital signal leading the second digital signal in time.
In yet another aspect of the present invention, the quadrature relationship is characterized by the second digital signal leading the first digital signal in time.
In accordance with another aspect of the present invention, a light source creates a light beam. A diffraction grating splits the light beam into at least at least a first light beam and a second light beam causing, respectively, the first light spot and the second light spot. Typically, the quadrature relationship is produced by adjusting the diffraction grating.
In still another aspect of the present invention, the photodetector unit forms a three-beam system comprising a first photodetector receiving the first light spot and a second photodetector receiving the second light spot.
In yet another aspect of the present invention, the photodetector unit forms a three-beam system and a first photodetector receives the first electrical signal being an E signal of the three-beam system and a second photodetector receives the second electrical signal being an F signal of the three-beam system.
In a further aspect of the present invention, a carriage moves a portion of the optical system across the optical medium. The carriage moves the optical system using one or more rates of motion. Each of the one or more rates of motion is determined to operate within an interval defining a distance between tracks of the optical medium. A first distance is computed to be a difference between a current track over which the optical system is presently positioned and a target track. The current track is determined from the up-count signal and the down-count signal. The optical system is moved at the rate of motion corresponding to the interval in which the first distance falls.
In an additional aspect of the present invention, the quadrature detector further comprises a first flip-flop having a clock input and a Q output. The first digital signal is coupled to the clock input of the first flip-flop. The up-count signal is coupled to the Q output of the first flip-flop. A second flip-flop has a clock input and a Q output. The second digital signal is coupled to the clock input of the second flip-flop. The up-count signal is coupled to the Q output of the second flip-flop.
In yet another aspect of the present invention, the digital shaping circuitry comprises a first Schmitt-trigger which converts the first electrical signal into the first digital signal and a second Schmitt-trigger which converts the second electrical signal into the second digital signal.
In still another aspect of the present invention, the optical system comprises an objective lens which directs the first light spot and the second light spot onto the optical medium.
In one embodiment of the present invention, a method conducts a high-speed search by adjusting at least a first light spot and a second light spot to form a quadrature relationship to each other. The first and second light spots are then directed onto an optical medium. The light spots traverse across tracks of the optical medium in one of an inward direction and an outward direction. A reflected component of the first light spot is received to form a first electrical signal and a reflected component of the second light spot is received to form a second electrical signal. The first electrical signal and the second electrical signal are shaped into a first digital signal and a second digital signal. From the quadrature relationship of the first digital signal and the second digital signal, an up-count signal indicating that the light spots are traversing the tracks in the first direction and a down-count signal indicating that the light spots are traversing the tracks in the second direction are determined.
In a further aspect of the present invention, the up-count signal and the down-count signal are counted to estimate a number of tracks traversed by the light spots.
In still another aspect of the present invention, a counter, having sufficient memory to record a maximum number of tracks traversed, receives the up-count signal and the down-count signals and generates the estimate of the number of tracks traversed.
In yet another aspect of the present invention, a microcomputer receives the up-count signal and the down-count signals and generates the estimate of the number of tracks traversed.
In another aspect of the present invention, the first light spot and the second light spot are moved at one of three rates of movement. One of the three rates of movement is selected based on a distance between a current track and a target track wherein the current track is estimated by the counting step.
In still another aspect of the present invention, a distance between a current track over which an optical system is presently positioned and a target track is determined. The optical system is configured to position the first and the second light spots on the optical medium. The optical system is moved at a high rate of movement if the distance is greater than a significant distance. The optical system is moved at a medium rate of movement if the distance is greater than a nominal distance but less than the significant distance. The medium rate of movement is less than the high rate of movement. The nominal distance is less than the significant distance. The optical system is moved at a low rate of movement if the distance is less than the nominal distance but greater than a minimal distance. The low rate of movement is less than the medium rate of movement and the minimal distance being less than the nominal distance. The number of tracks crossed is counted, based on the up-count signal and the down-count signal, while the optical system is moving according to one of the above moving steps, to determine the current track.
In yet another aspect of the present invention, a method determines a distance between a current track over which an optical system is presently positioned and a target track. The optical system is configured to position the first and the second light spots on the optical medium. A plurality of rates of motion is then determined for moving the optical system. A plurality of disjoint intervals defining a number of tracks to be crossed is determined. Each one of the plurality of rates of motion corresponds to one of the disjoint intervals. The optical system moves at one of the rates of motion if the distance falls within the corresponding interval. The number of tracks crossed is counted to determine the current track, based on the up-count signal and the down-count signal, while the optical system is moving according to one of the above moving steps.